In a previous life I used to sell mens clothing. Here is some advice for when you go for your fitting :

1- It's not the tailors fault. Don't take it out on him.
2- They've heard all of the jokes about monkey suits and penguins. Don't go there.
3- Wear a dress shirt with a collar to the fitting.
4- Take a pair of your own socks to the fitting also. You don't want to put on the socks that everyone else uses. Trust me.
5- Have it fitted as soon as possible. Bite the bullet. Get it over with.
6- If your niece gives you an option go for a single breasted tux with a shawl or notch lapel. They look the the best on a wide varity of body shapes.
7- If they put a wide pleated belt around your waist it's a cummerbund. The pleats go up. They were designed to hold theater tickets.
8- Spring for the formal shoes. Nothing looks worse than a tux with your dress shoes. Yes - it matters. The women will notice.

What's the plus side ?

9- Prepare for sex. If you can act casual as if you wear a tux all the time and don't tug at your collar like a school boy at his first dance the women will find you irresistible. They will want to touch you and take you home with them. Just try to refrain from saying "My name is Mario - M Mario".